Variable mutual double tuned i. f. transformer



July 3, 1956 R. M. ASHBY ETAL VARIABLE MUTUAL DOUBLE TUNED I.F.TRANSFORMER Filed April 1, 1953 FIG.2

FIG.

OOOOOOQ FIG.4

FIG. 3

INVENTORS ROBERT M. ASHBY EUGENE A. HOLMES III 774% C fink/Ins VARIABLEMUTUAL DOUBLE TUNED I. F.

' TRANSFORMER Robert M. Ashby, Pasadena, and Eugene A. Holmes III,Whittier, Calif.

Application April 1, 1953, Serial No. 346,292

1 Claim. (Cl. 336--75) (Granted under Title 35, U. S. Code (1952), see.266) This invention relates to improvements in high frequency resonantsystems such, for example, as those which are generally used foroperation at a particular frequency in radio frequency amplifiercircuits. In particular, this invention relates to a double tunedintermediate frequency transformer having a suitable band width forradar and television receivers.

In practice, where intermediate frequency amplifiers are operated in theregion of thirty megacycles with a desired band width as high as tenmegacycles, it is difiicult to obtain suificient coupling between theprimary and secondary windings of a conventional intermediate frequencytransformer to achieve the required band width and gain. Therefore, theusual practice has been to substitute an equivalent T or pi networkwhich is critical to design, adjust and align, since any change in anetwork component inherently affects the required values of the othercomponents.

The general purpose of the present invention is to provide a compact andflexible radio frequency transformer having a large coupling coefficientand an easily made adjustment of center frequency and band width.

Variable inductors having a fixed winding and a movable ferromageticcore for inductance adjustment have been in common use to obtainresonance at a predetermined frequency. Likewise, the idea of using linkcoupling between two separate coils has been often used to provide aninductively coupled circuit in which the effective mutual inductance isadjustable over a considerable range.

In the present invention, the primary and secondary windings of anintermediate frequency transformer are wound on D-shaped coil forms ofplastic material. Tuning is accomplished by adjusting the amount ofinsertion of Polyiron screws tapped into the coil forms, therebyproviding independent tuning adjustments for the primary and secondarywindings. The coil forms are mounted by screws to a metallic base whichalso supports a shield across its center which acts as an electrostaticshield, reducing the capacity coupling between the primary and secondarywindings. Coupling between these windings is obtained by means of ametal ring which is movable axially of the coil forms to interceptvarying amounts of the leakage flux.

The invention further resides in the features of construction andarrangement hereinafter described and claimed. For an understanding ofthis invention and illustrations of embodiments thereof, reference ismade to the accompanying drawings in which:

Fig. 1 is a perspective view of a preferred embodiment of an interstagetransformer constructed in accordance with the present invention;

Fig. 2 shows a cross section in elevation of the transformer of Fig. 1;

Fig. 3 shows a cross section in elevation of another embodiment of theinvention; and

Fig. 4 shows a section of the transformer of Fig. 3 taken at line 4-4.

f nited States Patent ice Referring to Fig 1, the primary winding 12 andthe secondary winding 13 of an interstage transformer 11 are woundrespectively on D-shaped plastic insulating coil forms 14 and 15 whichare fastened in a backtoback relationship to a supporting metallic basemember 16. Base member 16 also supports a metallic electrostatic shieldmember 17 across a diameter of the base between the coils, therebyproviding a grounded metallic insulation between the two Ds. The outeredges of shield member 17 extend beyond coil forms 14 and 15 and areformed with notches 18 spaced to form a particular screw thread.

Coupling between the primary winding 12 and a secondary winding 13 ismade by a link in the form'of ring 21. Referring to Fig. 2, ring 21 isseen to be of composite structure having an outer band 22 of a metalhaving good conductivity, such as aluminum, and an inner band 23 ofsuitable high frequency insulating material. The inner surface of band23 is provided with a screw thread 24 adapted to cooperate with thenotches 18 cut in shield member 17. Ring 21 is seen to be adjustableaxially of the coil forms 14 and 15 by a screwing motion imparted to thering to vary the amount of flux intercepted by the ring and, hence,thereby to adjust the amount of inductive coupling between the primaryand secondary windings 12 and 13. By utilizing D-shaped coils, thenonlinear portions of which form arcs of a circle that is concentricwith the coupling ring, rather than conventional coils of circularshape, higher coeflicients of coupling can be obtained since, with thisarrangement, more than half of the leakage flux of coils 12 and 13 canbe intercepted by link ring 21. This may be seen from the fact that thearcuated sides of coils 12 and 13 are of greater length than theircomplementary linear sides and from the fact that each half of ring 21cooperates with the total length of these arcuated sides.

Each coil form is also provided with a tapped opening 25 into whichthreaded cores 26 may be inserted. The threaded cores may be made ofmagnetic material, such as powdered iron, or of nonmagnetic material andmay be composed in part of a dielectric material, depending upon thefrequency at which the transformer is to operate. A preferred corematerial is known commercially as Polyiron, which consists of powderediron held in the desired shape by a plastic binder. Thus, the midbandfrequency of I. F. transformer 11 can be accurately determined byindependently adjusting the amount of insertion of cores 26 and the bandspread of this transformer can be precisely regulated by varying theaxial position of link coupling ring 21.

Fig. 3 illustrates a simplified version of the transformer of Fig. l inwhich the metallic base member and the electrostatic shield areeliminated. In this transformer two complementary D-shaped coil forms 32and 33, similar to coil forms 14 and 15 but having intermediate sections34 and 35 of reduced cross-sectional area on which the primary andsecondary windings 40 and 41 are wound, are held in a back-to-backrelationship by means of sleeve member 36. This sleeve may be made ofany suitable insulating material and its outer surface may be providedwith a screw thread similar to that carried by the inner surface ofmetallic ring member 37. With this arrangement, ring member 37 may bethreaded axially of windings 40 and 41 to intercept varying amounts offlux and to provide different coefficients of coupling. Since sleeve 36isolates ring member 37 from direct electrical contact with the primaryand secondary windings, this ring may be of complete metalliccomposition. Access to the primary and secondary windings is had viaport 38 formed in the lower portion of coil form 32. This transformerassembly may be conveniently mounted, if desired, by means of screwmembers 39 which cooperate with the screw threads cut in the coil forms32 and 33 for the slug tuning of the primary and secondarywinding. ifdesired, the screw threads maybe omitted from 1ements $6 and 37' and thevariable coupling obtained by slidahiy displacing the coupling tingalong the surface of the cylindrical sleeve member. The transformerofFig '2 possesses a relatively high coefficient of coup mg and thereforeit finds particular application in control cireuits where wide bandasscharacteristics are desired.

' Obviously many modifications and variations are possible in thelightof the above teachings; Itis therefore to be understood that withinthe sco e of the ap ended claims the invention may he practioe'dotherwise" than as specifically described.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for" overnmental-purposeswithout the payment of any royatties thereon or therefor.

is claimed is:- r I An intermediate fre uency transformer com risingincombination, a conductive base member; first" and sec: ond' D=shapedcoil forms attached to said base member With tliif" planar" SYi'if'Elsiii 5; 'jiiiilll fifi'ai'd' relationship and their nonlinear s i'lrfaceslying along the circumference of a circle, an electrostatic shieldmember having screw threads cut in a pair of opposite edges thereofdisposed between said surfaces and secured to said base member, primaryand secondary coil windings on said 'coil forms, and a conductive ringcircumventing said windings', said conductive ring having an inner bandof insulating material bearing a complementary screw thread whereby saidconductive ring may be axially moved along said shield member by athreading motion to intercept varyin amounts; of primar and secondaryleakage fluxand= whereby the coefiicient of coupling of said transformermay be adjusted.

References Cited in the file of this patent UNITED STATES PATENTS

